The present invention relates to a head cell apparatus used in wastewater treatment, and in particular, to an energy-efficient duct and methods used for conveying wastewater to be treated (i.e., influent) to the head cell.
One phase of wastewater treatment is separating “grit,” which is high-density, inorganic, settleable particles, from the influent. Grit causes wear to downstream treatment equipment and, if it accumulates, loss of performance.
One type of apparatus used for separating grit from influent is referred to as a head cell. Other approaches to removing grit have involved the use of a horizontal mechanically rotating element (e.g., a paddle or propeller) that circulates the influent within a surrounding cylindrical tank to separate the grit from the influent and cause it to gather in an accumulating well. By way of contrast, head cells separate grit by a continuous hydraulic action and do not require any mechanically-induced motion. Head cells are also self-cleaning.
Using a mechanically rotating element is disadvantageous because the periodic nature of its rotation creates turbulence that tends to re-suspend finer grit. Also, larger objects that are typically found in an influent flow, such as rags, as one example, can accumulate and “bridge” operating areas in the well. In this case, such an apparatus must then be drained, cleaned and/or repaired, which results in decreased treatment efficiency and increased operating costs.
The hydraulic separation action in a head cell occurs through controlling the influent to flow at predetermined speeds and along a predetermined course, and does not require the use of chemicals. The influent enters at the periphery or rim of a funnel-like conical surface from a direction tangential to the rim, and then flows over and around the downwardly sloping conical surface, at least partially circling a centrally located opening. The flow conditions are determined such that a dynamic boundary layer is developed at the conical surface.
As the influent flows around the downwardly sloping conical surface, the grit is separated out onto the conical surface. At the same time, the remaining liquid, i.e., the effluent (which is relatively grit-free wastewater) is guided to flow out of the head cell through openings located at the outer periphery of the conical surface. In general, this effluent is channeled for further treatment downstream, e.g., as primary sludge.
At the same time, the separated grit moves downwardly along the sloping conical surface and through the opening for collection at a point beneath the opening. A head cell may have several individual conical surfaces or “trays” that are vertically aligned with each other such that grit draining through the central opening in an upper tray also passes through similar central openings in all lower trays. In a typical head cell having vertically aligned or “stacked” trays, a greater working surface area is provided relative to the head cell's footprint than for comparably sized equipment having a single chamber with a mechanically rotating element.
In some head cell installations, referred to herein as “upward feed head cells,” the influent is pumped vertically upward such that each of the stacked trays, in succession from a bottom tray to a top tray, receives an amount of the influent through a peripheral inlet. The energy requirement of this arrangement can be high due to the loss of influent velocity head and the necessity of additional head to generate a suitable velocity in the peripheral inlet. In many installations, however, available head is limited, making this arrangement impractical.
Some wastewater treatment installations were originally implemented without grit removal equipment positioned upstream of the primary sludge treatment equipment. Retrofitting such installations with grit removal equipment is desirable to eliminate or at least reduce the amount of grit in the primary sludge before it enters the primary treatment equipment. Given the floor space constraints in existing installations, head cell equipment is often favored because it has a far superior capacity to remove grit (as great as 10 times more) per unit area of the equipment's footprint than the mechanically rotating element design. These retrofit installations, however, often have the same energy limitations that prevent use of an upward feed head cell.